1. Field of the Invention
The present invention relates to a navigation device for a vehicle, a method for producing data for a search, and a method for searching for a guided route. More specifically, the present invention relates to a navigation device for a vehicle, a method for producing data for a search, and a method for searching for a guided route that displays the guided route and guides a driver to a destination.
2. Description of the Prior Art
Typical navigation devices for vehicles display a position of a vehicle, and a guided route for use in guiding the vehicle to a destination on map data.
In order to provide these functions, the navigation device generally defines a branch point of a road as a node, and defines a road connecting nodes as a road link. The navigation device stores a set of nodes and road links as map data.
The navigation device additionally identifies an intersection of various branch roads in the map data, also referred to as a crossroad, by adding an attribute representing the crossroad to nodes. Further, according to Japanese Laid-Open Patent Publication (Kokai) No. 2004-117268, the navigating device may identify a roundabout by adding an attribute representing a roundabout to nodes and road links in the map data.
A position of a vehicle is shown on a road map based on various data, such as the map data described above. A driver may input multiple names of roads from input means, and the navigation device may search for the nodes and road links constituting these multiple roads that were input by the driver based on the map data. Then, the navigation device may search for a node at which multiple roads join, and which has the attribute of a crossroad or a roundabout as a destination or a via-point that the vehicle travels through. Further, a map including the searched point may be displayed to the driver for guiding to the vehicle to the destination.
However, in reality, criterion for defining a branch road as a crossroad or a roundabout depends on map companies, which may not define the a crossroad or a roundabout in the same way. For example, map companies may not consistently define a case where two T-intersections are located adjacent to each other, and are connected by another road, referred to as a dogleg branch road hereinafter; a case of a roundabout with a diameter equal to or less than 25 m; a case of a roundabout which is not one-way; a case of a roundabout to which another link such as a road or a railroad is connected in the middle thereof; or a case of a roundabout which includes a traffic light. As a result, these cases cannot be shown as destinations or via-points.
Moreover, even if multiple names of roads are entered, when these roads do not include crossroads and roundabouts, the search fails, and a map including the multiple roads and surrounding areas thereof is not shown. However, even if multiple roads are not connected with other, when a driver heads in the same direction to which he or she wants to go, it is possible to take the same road to reach a neighborhood of a final destination.
These problems prevent providing finer route guidance.